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Author Topic:   Battery selector switches
frontier posted 11-03-2007 06:29 PM ET (US)   Profile for frontier   Send Email to frontier  
For years I've thought and been told never to switch the battery selector switch while the engine is running. It would damage the alternator. Just read an article in "Boat US" magazine that says it's OK as long as you don't switch it to "OFF". Makes sense. Good explanation. No problem because whenever it's switched to "1", "ALL" or "2", it's connected to a battery at all times.
swist posted 11-04-2007 12:19 PM ET (US)     Profile for swist  Send Email to swist     
Yes, it's the OFF position that is the problem while running. Can blow the alternator diodes.

A good switch will make sure that at least one battery is always connected when you move between 1-2-both, but there is obviously a higher risk that when the switch contacts are in motion that might be a brief disconnect of all batteries, certainly most likely if the switch is older, and/or starting to corrode, and/or of less than optimal design.

davej14 posted 11-04-2007 07:46 PM ET (US)     Profile for davej14  Send Email to davej14     
This makes sense if the switches are designed with a "make before break" contact system. A quick check of the Perko catalog revealed that indeed this is the case for at least some of their switches (I did not check them all). You would need to make sure that you always pass through the "ALL" position when switching between batteries for this to be effective. I also suspect that some motors are more tolerant of switching off a battery connection since they are designed to be pull started without a battery in the first place.

Of greater concern would be if the electronics are wired in a fashion that they would be connected to the motors alternator without the battery connected. There is no inherent reason that a battery needs to be in the circuit for proper voltage regulation, it would be dependent upon the specific motor.

jimh posted 11-04-2007 09:34 PM ET (US)     Profile for jimh  Send Email to jimh     
Given the cost and difficulty of repairing the outboard motor charging system, I just refrain from changing the position of the battery switch while the engine is running.
contender posted 11-06-2007 10:21 PM ET (US)     Profile for contender  Send Email to contender     
swist is correct, switch the switch there goes your diodes. Why would you want to turn off your switch after the engine is running anyway?
davej14 posted 11-08-2007 10:08 AM ET (US)     Profile for davej14  Send Email to davej14     
In the prior original posts, no one is suggesting that you turn off the battery connection, in fact it is specifically stated that this should not be done. The question was if switching from Bat 1 to "both" to Bat 2 would be OK.

I could see this as desirable if you started on one battery and then wanted to charge both batteries while running without first shutting off the motor.

swist posted 11-09-2007 09:02 AM ET (US)     Profile for swist  Send Email to swist     
How much trouble is it to stop the motor for 20 seconds just to be absolutely sure you don't screw up (by accidentally tuning to OFF), or the switch doesn't screw up (by having the supposed-make-before-break contacts lift while rotating).
jimh posted 11-09-2007 09:30 AM ET (US)     Profile for jimh  Send Email to jimh     
When you move the battery selector switch while the engine is operating, you change the load on the engine charging system. If the change in load is small, this is not a problem, however, if the change in load is very large, there could be transient voltages generated. If one of the batteries in a two-battery system is completely dead, or if the battery has a shorted cell, connecting or disconnecting it to the charging system while the engine is running will result in a big change in the load on the charging system. And this could generate some transient voltage swings.

In exchange for the convenience of moving the battery switch while the engine is operating one has to consider the possible risk of damage and the cost to repair that damage. My analysis of the problem is that the cost of the possible damage is rather high and the value of the convenience of moving the switch while the engine is running is low. In weighing those two factors, I have come to the conclusion that even though it may be perfectly reasonable in theory to move the battery switch while the engine is running, I tend to avoid doing it because it does not seem to be worth the risk of possible damage.

dino54904 posted 12-27-2007 10:12 AM ET (US)     Profile for dino54904  Send Email to dino54904     
Okay...but...if you have your selector switch on Bat #1 and you left some electronics or lights on that drained that battery so now it won't start an engine. You switch to Bat #2 and start the engine. Can't you simply switch back to Bat #1 while the engine in running to recharge that battery that was drained??? Will this risk harming the engine and if so this takes away one of the main benefits of having a dual battery system. OR...would you keep the selector on Bat #2...start the engine and keep the selector on that battery for that trip and recharge Bat #1 with a shore powered electric charger?
Bella con23 posted 12-27-2007 10:00 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
Dino - You have the picture. If you want to eliminate any possibility of damaging the charging circuit components, you must follow the advice of starting on the charged battery and not moving the selector switch until you return to the dock for a shore charge.

Personally, as I have mentioned on a previous post, I use
"A" battery out to my destination and switch to "B" battery on the way back. With 30 years of boating under my belt, I don't concern myself with whether or not the engine is running when I make the switch.

It should be noted that I would always make it back to safe harbor if my charging system should fail.

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